Those Inches Matter

The single-action Army in .45 Colt in GURPS.

The M1873 "Cavalry" model does 3d-2 pi+ with it's 7-1/2" barrel.  8 raw, 12 after damage type average.  1-16 (1-24) spread.
The "Artillery" model's 5-1/2" barrel does 2d+2 pi+.  9 raw, 13 after damage type average.  4-14 (6-21) spread.
The ubiquitous "Civilian" model with a 4-3/4" barrel does 2d+1 pi+.  8 raw, 12 after damage type average.  3-13 (4-19) spread.

Going even shorter loses the ejector and makes the reload 4i instead of 3i.

The "Storekeeper's" model and it's 4" barrel sends 2d pi+.  7 raw, 10 after damage type average.  2-12 (3-18) spread.
The stubby 2-1/2" to 3" barreled "Sheriff's" mode drops to 2d-1 pi+.  6 raw, 9 after damage type average.  1-11 (1-16) spread.

The official stats didn't deign to give us different ranges for these, all are shown with the default 120/1,300 from the 7-1/2" Cav gun.  We do get changes to weight and bulk.

But, WAIT! they get longer too!

The 10" gun has the same stats as a 7-1/2" except for weight and bulk.

The 16" gun gives 3d-1 pi+ for damage, also with and increase in weight and bulk.  9 raw, 13 after damage type average.  2-17 (3-25) spread.

Historically, there don't appear to be any 12" "Buntline Specials"!  It's such a staple of fiction too.

Using .45 S&W in any of these guns is a -2 to damage.  From 1875 .45 S&W is the issue ammo for all .45 revolvers, rationalizing the supply chain with the S&W No.3 M1875 "Schofield".

Normal .45 Colt ammunition was still found in the trooper's ammunition pouches because it could be bought from the sutler's station and other commercial sources, such as Sear's.